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Date: 23 Sep 93 23:51:54 GMT
From: eachus@mitre-bedford.arpa  (Robert I. Eachus)
Subject: Re: null unconstrained arrays vs generic formal types
Message-ID: <EACHUS.93Sep23185154@spectre.mitre.org>
List-Id: <comp.lang.ada>

In article <23SEP199311043613@bambam.gsfc.nasa.gov> nbssal@bambam.gsfc.nasa.gov
 (Stephe Leake) writes:


   > So, does Ada 9x fix this by relaxing LRM 12.3.4(5)?  Does anyone
   > have an alternate solution?

   It took me a while to recognize why you had a problem. (Even if you
quoted my favorite paragraph number in Ada 83.)  Ada 9X retains the
same rule, although you could submit this as a comment on the standard
during the balloting.  Of course, the "right" solution might be to
have generic formal subtypes.  But I don't think you need any of that.

   In Ada 9X you could make the array type a generic formal derived
type, but the real solution is to reorganize things so that you have a
generic formal package:

   generic
      type Index_Base_Type is range <>;
      type Element_Type is range <>; -- why not private?
      Index_Last: Index_Base_Type;
   package Element_Arrays is
       subtype ZERO_INDEX_TYPE is range 0 .. Index_Last;
       subtype INDEX_TYPE is ZERO_INDEX_TYPE range 1 .. Index_Last;
       type INDEX_ARRAY_ELEMENT_TYPE is array (INDEX_TYPE range <>)
	   of ELEMENT_TYPE;
        procedure Element_Get
	   (Item : out ELEMENT_TYPE; Width : in Text_IO.FIELD);
        procedure Element_Put (
	   Item : in ELEMENT_TYPE;
	   Width : in Text_IO.FIELD;
	   Base  : in Text_IO.NUMBER_BASE);
        ...
    end Element_Arrays;

    package Int_30_Arrays is new Element_Arrays(Integer, Integer, 30);
    -- for example...

    Now your generic array IO package looks like:

   with Text_IO;
   generic
      with package Elem_Arrays is new Element_Arrays(<>);
   package Unconstrained_Integer_1D is
       use Elem_Arrays; -- depending on your religion. :-)
       procedure Get (Item : out INDEX_ARRAY_ELEMENT_TYPE; Last : out ZERO_INDE
X_TYPE);
       procedure Put (Item : in INDEX_ARRAY_ELEMENT_TYPE);
   end Unconstrained_Integer_1D;

   Of course you could put Element_Get and Element_Put elsewhere, but
the principle is that you can pass two subtypes of the same type as
generic parameters in Ada 9X by wrapping them in a generic package
parameter.
--

					Robert I. Eachus

with Standard_Disclaimer;
use  Standard_Disclaimer;
function Message (Text: in Clever_Ideas) return Better_Ideas is...